The effects of a pre- and postnatal hypothyroid environment on the development of the neocortex have been determined. Rats were treated with propylthiouracil (PTU) with or without thyroid hormone supplementation during pregnancy and throughout the early development of the pups. The Cavalieri method was used to provide an estimate of the volume of the neocortex, and a combination of the Cavalieri method and the ‘disector’ particle-counting method provided an unbiased estimate of the numbers of glia and neurons in the neocortex. In pups from non-treated rats the mean volume of the neocortex increased from 31.3 mm3 at 5 days postnatally to 191.5 mm3 at 20 days and then remained constant to day 48. Similarly, the mean number of glial cells increased from 5.2 × 106 at day 5 to 12.0 × 106 on day 20 and then remained constant to day 48. The mean number of neurons in the neocortex in the control condition was constant at about 14.6 × 106 from day 5 to day 48. PTU treatment during pregnancy and postnatal development significantly decreased the mean volume of the neocortex at all of the stages studied, for example from 31.3 to 23.3 mm3 (p < 0.001) on day 5 and from 191.5 to 155.0 mm3 (p < 0.001) on day 20. Supplementation with thyroxine (T4) or tri-iodothyronine partially reversed this at certain stages. For example, the mean volume of the neocortex at 48 days was 197.5 mm3 in control rats, 118.1 mm3 in PTU-treated rats and 169.3 mm3 in PTU-treated rats supplemented with T4. Within the neocortex, the volume of the neuropile was more severely affected than was the volume of the neuron cell bodies. For example, at 20 days the mean volume of the neuropile in control rats was 160.2 mm3; in PTU-treated rats the mean volume was 125.8 mm3 (p < 0.001). PTU reduced the number of glial cells in the neocortex; this effect was statistically significant in the 48-day pups (controls = 11.8 × 106, PTU-treated = 8.6 × 106, p < 0.05). Treatment with PTU also reduced the mean number of neurons in the neocortex, although not to a statistically significant degree, and supplementation with T4 ameliorated this effect. These observations are consistent with the idea that normal thyroid hormone levels are required for the early growth and maturation of the neocortex.